Image sensor and method of forming the same

ABSTRACT

An image sensor includes a photodiode formed in a substrate, a buffer oxide layer, a blocking layer, an upper oxide layer, and a transmission supplementary layer. The buffer oxide layer covers the photodiode, and the blocking layer is disposed on the buffer oxide layer to cover the photodiode. The upper oxide layer covers the blocking layer, and the transmission supplementary layer is interposed between the upper oxide layer and the buffer oxide layer to cover the photodiode. The transmission supplementary layer has a refractive index between the refractive index of the blocking layer and at least one reflective index selected from the refractive indexes of the buffer oxide layer and upper oxide layer.

CROSS-REFERENCE TO RELATED APPLICATION

This application claims priority under 35 U.S.C. § 119 to Korean PatentApplication No. 2004-44049, filed on Jun. 15, 2004 in the KoreanIntellectual Property Office, the disclosure of which is incorporated byreference herein in its entirety.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a semiconductor device and a method offorming the same. More specifically, the present invention is directedto an image sensor and a method forming the same.

2. Description of the Related Art

The image sensor of a semiconductor device converts externally incidentlight into electrical signals. A pixel of the CMOS image sensor includesa light receiver and a CMOS logic section. The light receiver sensesexternal light, and the CMOS logic section converts signal chargesgenerated from the light receiver by the sensed light into electricalsignals (i.e., data).

The light receiver of a typical CMOS image sensor uses a photodiode. Ifthe external light impinges on the photodiode, electron-hole pairs areproduced in the photodiode to generate signal charges. The generatedsignal charges are accumulated in the photodiode. The accumulated signalcharges are converted into data by handling in the CMOS logic section.

Various studies have been performed to enhance the dynamic range of theCMOS-type image sensor. The dynamic range of a CMOS-type image sensormay be lowered by many factors such as, for example, dark currentgenerated by the dangling bond of the substrate surface where thephotodiode is formed. Further, metallic elements of the metal layerformed on the top surface of the photodiode may penetrate the photodiodethrough the oxide layer to generate dark current. Thus, the dark currentmay lower the dynamic range of the image sensor.

Various efforts have been made to reduce dark currents. One of theseefforts will now be described in FIG. 1.

Referring to FIG. 1, a device isolation layer 2 is disposed at apredetermined region of a semiconductor substrate 1 to define a dioderegion. A P-type well is formed in the diode region “a”. An N-typephotodiode 3, a region doped with N-type impurities, is formed in thediode region “b”. The N-type photodiode 3 and the P-type well constitutea PN junction. A P-type photodiode 4 is formed at a surface of the dioderegion on the N-type photodiode 3. One side of the P-type photodiodeextends to be electrically connected to the P-type well. A siliconnitride layer 6 covers the diode region. A buffer oxide layer 5 made ofsilicon oxide is interposed between the silicon nitride layer 6 and thesurface of the diode region. An upper oxide layer 7 is disposed on thesilicon nitride 6. The upper oxide layer 7 may include interlayer oxideand may be made of silicon oxide.

In the conventional image sensor, the P-type photodiode 4 can suppressthe dark current generated by the dangling bond distributed at theinterface of the diode region and the buffer oxide layer 5. That is,among electron-hole pairs, electrons are coupled with holes in theP-type photodiode 4 and the holes are emitted at the P-type well toreduce dark current generated by the dangling bond. The silicon nitridelayer 6 prevents metallic elements of the overlying metal layer (e.g., ametal layer for forming an interconnection or a metal layer for formingsilicide) from penetrating the photodiodes 3 and 4. Thus, the siliconnitride 6 makes it possible to generate dark currents when the metallicelements penetrate the photodiodes 3 and 4.

The conventional image sensor includes a buffer oxide layer 5, a siliconnitride layer 6, and an upper oxide layer 7 which are sequentiallystacked on the photodiodes 3 and 4. For this reason, an externallyincident light impinges on the photodiodes 3 and 4 through the upperoxide layer 7, the silicon nitride layer 6, and the buffer oxide layer5. The impinging light may be partly reflected from the interfaces oflayers 5, 6, and 7. That is, the impinging light may be lost throughlayers 5, 6, and 7 due to its reflection. As a result, light finallyreaching the photodiodes 3 and 4 may be lost due to the lowerphotosensitivity of the image sensor, as compared to the incident light.

SUMMARY OF THE INVENTION

Exemplary embodiments of the present invention are directed to an imagesensor for enhancing the transmissivity of the externally incident lightand a method of forming the same.

Exemplary embodiments of the present invention are directed to an imagesensor for enhancing photosensitivity and a method of forming the same.

In an exemplary embodiment, there is provided an image sensor. The imagesensor includes a photodiode, a buffer oxide layer, a blocking layer, anupper oxide layer, and a transmission supplementary layer which areformed in a substrate. The buffer oxide layer covers the photodiode, andthe blocking layer is disposed on the buffer oxide layer to cover thephotodiode. The upper oxide layer covers the blocking layer. Thetransmission supplementary layer is interposed between the upper oxidelayer and the buffer oxide layer to cover the photodiode. Thetransmission supplementary layer has a refractive index between arefractive index of the blocking layer and at least one reflective indexselected from the reflective indexes of the buffer oxide layer and upperoxide layer.

In some embodiments, the buffer and upper oxide layers are preferablymade of silicon oxide and the blocking layer is preferably made ofsilicon nitride. The transmission supplementary layer is preferably madeof an insulator having a higher refractive index than the silicon oxideand a lower refractive index than the silicon nitride. The transmissionsupplementary layer is preferably made of silicon oxynitride. Thephotodiode may include an N-type photodiode formed in the substrate anda P-type photodiode formed at a surface of the substrate on the N-typephotodiode. The transmission supplementary layer may be interposedbetween the blocking layer and the upper oxide layer. Alternatively, thetransmission supplementary layer may be interposed between the bufferoxide layer and the blocking layer. Alternatively, the transmissionsupplementary layer may include a first transmission supplementary layerinterposed between the buffer oxide layer and the blocking layer and asecond transmission supplementary layer interposed between the blockinglayer and the upper oxide layer.

In another exemplary embodiment, there is provided a method of formingan image sensor. The method includes forming a photodiode in a substrateand forming a buffer oxide layer to cover the photodiode. A blockingoxide layer is formed on the buffer oxide to cover the photodiode. Anupper oxide layer is formed to cover the blocking layer. A transmissionsupplementary layer is interposed between the upper oxide layer and thebuffer oxide layer to cover the photodiode. The transmissionsupplementary layer has a refractive index between a refractive index ofthe blocking layer and at least one reflective index selected from thereflective indexes of the buffer oxide layer and the upper oxide layer.

In some embodiments, the buffer oxide layer and upper oxide layer arepreferably made of silicon oxide and the blocking layer is preferablymade of silicon nitride. The transmission supplementary layer ispreferably made of an insulator having a higher refractive index thanthe silicon oxide and a lower refractive index than the silicon nitride.The transmission supplementary layer may be made of silicon oxynitride.The formation of the photodiode includes forming an N-type photodiode ina predetermined region of the substrate and forming a P-type photodiodeat a surface of the substrate on the N-type photodiode. The formation ofthe transmission supplementary layer and the blocking layer includesforming the blocking layer on the buffer oxide layer and forming thetransmission supplementary layer on the blocking layer. Alternatively,the formation of the transmission supplementary layer and the blockinglayer includes forming the transmission supplementary layer on thebuffer oxide layer and forming the blocking layer on the transmissionsupplementary layer. Alternatively, the formation of the transmissionsupplementary layer and the blocking layer includes forming a firsttransmission supplementary layer on the buffer oxide layer to cover thephotodiode, forming a blocking layer on the first transmissionsupplementary layer, and forming a second transmission supplementarylayer on the blocking layer to cover the photodiode. The transmissionsupplementary layer includes the first and second transmissionsupplementary layers.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a plan view of a light receiver of a conventional imagesensor;

FIG. 2 is a cross-sectional view of an image sensor according to anembodiment of the present invention;

FIG. 3 and FIG. 4 are cross-sectional views for explaining the method offorming the image sensor shown in FIG. 2;

FIG. 5 is a cross-sectional view of an image sensor according to anotherembodiment of the present invention;

FIG. 6 and FIG. 7 are cross-sectional views for explaining the method offorming the image sensor shown in FIG. 5;

FIG. 8 is a cross-sectional view of an image sensor according to stillanother embodiment of the present invention; and

FIG. 9 and FIG. 10 are cross-sectional views for explaining the methodof forming the image sensor shown in FIG. 8.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

The present invention will now be described more fully hereinafter withreference to the accompanying drawings, in which preferred embodimentsof the invention are shown. The invention may, however, be embodied indifferent forms and should not be construed as limited to theembodiments set forth herein. Rather, these embodiments are provided sothat this disclosure will be thorough and complete, and will fullyconvey the scope of the invention to those skilled in the art. In thedrawings, the height of layers and regions are exaggerated for clarity.It will also be understood that when a layer is referred to as being“on” another layer or substrate, it can be directly on the other layeror substrate, or intervening layers may also be present. Like numbersrefer to like elements throughout.

Embodiment 1

FIG. 2 is a cross-sectional view of an image sensor according to anembodiment of the present invention.

Referring to FIG. 2, a device isolation layer 102 is disposed in apredetermined region of a semiconductor substrate (hereinafter referredto as “substrate”) 100 to define a diode region “a” and a transistoractive region “b”. The transistor active region “b” is connected to oneside of the diode region “a”. Field effect transistors included in aCMOS logic section of a CMOS-type image sensor may be formed in thetransistor active region “b”.

A P-type well is formed in the diode region “a” and the transistoractive region “b”. An N-type photodiode 104 is disposed in the dioderegion “b”. The N-type photodiode 104 and the P-type well constitute aPN junction. A P-type photodiode 106 is disposed at a surface of thediode region “a” on the N-type photodiode 104. One side of thephotodiode 106 extends laterally to be electrically connected to theP-type well.

A gate electrode 112 is disposed on the transistor active region “b”adjacent to the diode region “a”. A gate insulation layer 110 isinterposed between the gate electrode 112 and a surface of thetransistor active region “b”. Although not shown in this figure, othergate electrodes may be sequentially disposed in the transistor activeregion “b”. The gate electrode 112 may be made of a conductive material(e.g., doped polysilicon or polycide) or a conductive metal containingmaterial. The gate insulation layer 110 may be made of silicon oxide,particularly, thermal oxide. An impurity-doped layer 127 is disposed inrespective active regions “b” adjacent to opposite sides of the gateelectrode 112. The impurity-doped layer 127 may be doped with P-typeimpurities. The impurity-doped layer 127 may include a lightly dopedlayer 114 and a heavily doped layer 126. As illustrated in this figure,the impurity-doped layer 127 may have a double doped drain structure(DDD) structure in which the lightly doped layer 114 surrounds theheavily doped layer 126. Alternatively, the impurity-doped layer 127 mayhave a lightly doped drain (LDD) structure. The impurity-doped layer 127may corresponds to a floating diffusion layer of a CMOS-type imagesensor, and the gate electrode 112 may correspond to a transfertransistor of a CMOS-type image sensor.

A channel doping layer 108 may be disposed on a surface of thetransistor active region “b” below the gate electrode 112. The channeldoping layer 108 may be doped with the same impurities as the P-typephotodiode 106, i.e., P-type impurities. The channel doping layer 108may be omitted.

The blocking pattern 118 a covers the diode region “a”. That is, theblocking pattern 118 a covers the photodiodes 104 and 106. The blockingpattern 118 a may laterally extend to partially cover one sidewall and atop surface of the gate electrode 112. A buffer oxide layer 116 may beinterposed between the blocking pattern 118 a and a surface of the dioderegion “a”. The buffer oxide layer 116 may laterally extend to cover asidewall and a top surface of the gate electrode 112 and the surface ofthe transistor active region “b”.

The blocking pattern 118 a may be made of an insulator to preventpenetration of metallic elements. For example, the blocking pattern 118a is preferably made of silicon nitride. The buffer oxide layer 116 ismade of a material to reduce a tensile stress between the blockingpattern 118 a and the surface of the diode region “a”. For example, thebuffer oxide layer 116 is preferably made of silicon oxide. Due to thebuffer oxide layer 116, the tensile stress between the blocking pattern118 a and the surface of the diode region “a” is reduced to preventdamage on the surface of the diode region “a”.

An upper oxide layer 135 is disposed to cover the blocking pattern 118a. The upper oxide layer 135 may cover an entire surface of thesubstrate 100. The upper oxide layer 135 is preferably made of siliconoxide. The upper oxide layer 135 may include at least one interlayeroxide. The upper oxide layer 135 may include oxide having a differentfunction, i.e., prevent a silicidation.

The transmission supplementary pattern 120 a is interposed between theblocking pattern 118 a and the upper oxide layer 135. Preferably, thetransmission supplementary pattern 120 a has a sidewall aligned to asidewall of the blocking pattern 118 a. The transmission supplementarypattern 120 a covers the photodiodes 104 and 106. Preferably, the bottomsurface of the transmission supplementary pattern 120 a is in directcontact with the blocking pattern 118 a and the top surface of thetransmission supplementary pattern 120 a is in direct contact with abottom surface of the upper oxide layer 135.

The transmission supplementary pattern 120 a is made of an insulatorhaving a refractive index between a refractive index of the blockingpattern 118 a and a refractive index of the upper oxide layer 135.Particularly, if the blocking pattern 118 is made of silicon nitride andthe upper oxide layer 135 is made of silicon oxide, the transmissionsupplementary pattern 120 a is preferably made of an insulator having arefractive index higher than silicon oxide and lower than siliconnitride. Preferably, the transmission supplementary pattern 120 a ismade of, for example, silicon oxynitride.

A gate spacer 124 is disposed on one sidewall of the gate electrode 112adjacent to the impurity-doped layer 127. Preferably, the spacer 124includes a first spacer 118 a and a second spacer 120 b which arestacked in the order named. The first spacer 118 b is preferably made ofthe same material as the blocking pattern 118 a, and the second spacer120 b is preferably made of the same material as the transmissionsupplementary pattern 120 a.

In the above-described image sensor, the refractive index of thetransmission supplementary pattern 120 a ranges from the refractiveindex of the upper oxide layer 135 to the refractive index of theblocking pattern 118 a. That is, a difference between the refractiveindexes of the upper oxide layer 135 and the blocking pattern 118 a isreduced by the transmission supplementary pattern 120 a to enhance atransmissivity of an external light passing the layers 116, 118 a, 120a, and 135 stacked on the photodiodes 104 and 106. In other words, aloss of the external light is minimized by the layers 116, 118 a, 120 a,and 135 to enhance a photosensitivity of the image sensor.

A method of forming the image sensor shown in FIG. 2 will now bedescribed with reference to FIG. 3 and FIG. 4.

Referring to FIG. 3, a device isolation layer 102 is formed in apredetermined region of a substrate 100 to define a diode region “a” anda transistor active region “b”. The transistor active region “b” isconnected to one side of the diode region “a”. A P-type well is formedin the diode region “a” and the transistor active region “b”. Theformation of the P-type well may be performed using impurityimplantation. The P-type well may be formed after or before formation ofthe device isolation layer 102.

Impurities are selectively implanted to form an N-type photodiode 104 inthe diode region “a”. Impurities are selectively implanted into asurface of the diode region “a” on the N-type photodiode 104 to form theP-type photodiode 106.

Preferably, one side of the photodiode 106 is electrically connected tothe P-type well.

The channel doping layer 108 may be formed on a surface of thetransistor active region “b” adjacent to the diode region “a”. Theformation of the channel doping layer 108 may be performed by implantingP-type impurities. In some cases, the P-type photodiode 106 and thechannel doping layer 108 may be formed at the same time.

The gate insulation layer 110 and the gate conductive layer aresequentially formed on an entire surface of the substrate 100. The gateconductive layer is patterned to form a gate electrode 112 covering thechannel doping layer 108. The gate insulation layer is interposedbetween the gate electrode 112 and the transistor active region “b”. Thegate insulation layer 110 may be made of silicon oxide, particularly,thermal oxide. The gate electrode 112 may be made of a conductivematerial (e.g., doped polysilicon or polycide) or a conductive metalcontaining material.

Impurities are selectively implanted to form a lightly doped layer 114in the transistor active region disposed at opposite sides adjacent tothe photodiodes 104 and 106. The lightly doped layer 114 may be dopedwith P-type impurities.

The gate insulation layer 110 formed at the diode region “b” disposed atopposite sides adjacent to the gate electrode and on the surface of thetransistor active region “b” may be removed using a wet etch afterformation of the lightly doped layer 114.

The buffer oxide layer 116 is formed on an entire surface of a substrate100 including the lightly doped layer 114. Preferably, the buffer oxidelayer 116 is made of silicon oxide. The buffer oxide layer 116 may bemade of thermal oxide or silicon oxide based on chemical vapordeposition (CVD). The blocking layer 116 and the transmissionsupplementary layer 120 are sequentially formed on the buffer oxidelayer 116. They are conformally formed. Preferably, the blocking layer118 is made of an insulator to prevent penetration of metallic elements,for example, silicon nitride.

The photoresist pattern 122 is formed on the transmission supplementarylayer 120 to cover the diode region “a”. Thus, the transmissionsupplementary layer 120 formed on the transistor active region “b” isexposed. The photoresist pattern 122 may laterally extend to cover onesidewall and a top surface of the gate electrode 112 adjacent to thediode region “a”.

Referring to FIG. 4, using the photoresist pattern 122 as an etch mask,the transmission supplementary layer 120 and the blocking layer 118 aresuccessively anisotropically etched to form the blocking pattern 118 aand the transmission supplementary pattern 120 a which are sequentiallystacked on the diode region “a” and to form the gate spacer 124 on onesidewall of the gate electrode 112. In this case, the buffer oxide layer116 may act as an etch-stop layer. The gate spacer 124 includes a firstspacer 118 b and the second spacer 120 b which are stacked in the ordernamed. The transmission supplementary pattern 120 a is made of the samematerial as the second spacer 120 b, and the blocking pattern 118 a ismade of the same material as the first spacer 120 b.

Following formation of the gate spacer 124, the photoresist pattern 122is removed.

Using the gate spacer 124 as a mask, impurities are selectivelyimplanted to form the heavily doped layer 126. The lightly and heavilydoped layers 114 and 126 constitute the impurity-doped layer 127. Theimpurity-doped layer 127 is formed in the transistor active region “b”disposed at opposite sides adjacent to the photodiodes 104 and 106. Theimpurity-doped layer 127 may correspond to a floating diffusion layer ofa CMOS-type image sensor. The impurity-doped layer 127 may have an LDDor DDD structure.

The upper oxide layer 135 is formed to cover the transmissionsupplementary pattern 120 a. Preferably, the upper oxide layer 135 ismade of silicon oxide. The transmission supplementary pattern 120 a ismade of an insulator having a refractive index between a refractiveindex of the upper oxide layer 135 and a refractive index of theblocking pattern 118 a. Particularly, if the upper oxide layer 135 ismade of silicon oxide and blocking pattern 118 a is made of siliconnitride, the transmission supplementary pattern 120 a is preferably madeof an insulator having a refractive index higher than the silicon oxideand lower than the silicon nitride. Preferably, the transmissionsupplementary pattern 120 a is made of, for example, silicon oxynitride.

The upper oxide layer 135 may include the silicide barrier layer 129,which is made of silicon oxide. The silicide barrier layer 129 coversthe transmission supplementary pattern 120 a, and may cover the gateelectrode 112 and the impurity-doped layer 127. The silicide barrierlayer 129 may prevent metal silicide form being formed on a surface ofthe impurity-doped layer 127. The upper oxide layer 135 may include atleast one interlayer oxide layer formed on the silicide barrier layer129. In FIG. 4, there are shown first and second interlayer oxide layers131 and 132. They are made of silicon oxide.

Although not shown in this figure, a passivation layer may be formed onthe upper oxide layer 135.

In the above-described method, while the blocking pattern 118 a and thetransmission supplementary pattern 120 a are formed, the photodiodes 104and 106 are not exposed to plasma used in an etch process. Thus, thephotodiodes 104 and 106 may be protected from plasma damage to preventdegradation of the photodiodes 104 and 106.

The transmission supplementary pattern 120 a is formed between theblocking pattern 118 a and the upper oxide layer 135 to reduce thedifference between refractive indexes of the upper blocking oxide layer135 and the blocking pattern 118 a. Thus, a transmission coefficient ofan externally incident light increases to enhance a photosensitivity ofthe image sensor.

Embodiment 2

In another embodiment, a modified version of the transmissionsupplementary layer according to the first embodiment will now bedescribed. Therefore, same numerals denote same elements throughout thefirst and second embodiments.

FIG. 5 is a cross-sectional view of an image sensor according to anotherembodiment of the present invention.

Referring to FIG. 5, a device isolation layer 102 is disposed in asubstrate 100 to define the diode region “a” and the transistor activeregion “b”. The transistor active region “b” is connected to the dioderegion “a”. The P-type well may be disposed in the diode region “a” andthe transistor active region “b”. As previously stated in the firstembodiment, N- and P-type photodiodes 104 and 106 are disposed in thediode region “a” and the gate electrode 112 is disposed over thetransistor active region “b” with the gate insulation layer 110interposed therebetween. The channel doping layer 108 may be disposed onthe surface of the transistor active region “b” below the gate electrode112.

Blocking pattern 218 a is disposed to cover the diode region “a”. Theblocking pattern 218 a covers the photodiodes 104 and 106. Preferably,the blocking pattern 218 a is made of an insulator to preventpenetration of metallic elements. Therefore, the blocking pattern 218 ais preferably made of, for example, silicon nitride. The blockingpattern 218 a may laterally extend to partially cover one sidewall and atop surface of the adjacent gate electrode 112. The buffer oxide layer116 is interposed between the blocking pattern 218 a and a surface ofthe diode region “a”. Preferably, the buffer oxide layer 116 is made ofsilicon oxide. The buffer oxide layer 116 may laterally extend to covera surface of the gate electrode and a surface of the active region “b”.The upper oxide layer 135 covers the blocking pattern 218 a. Preferably,the upper oxide layer 135 is made of silicon oxide. As previouslydescribed in the first embodiment, the upper oxide layer 135 may includeat least one interlayer oxide or oxide of another function.

The first transmission supplementary pattern 216 a is interposed betweenthe blocking pattern 218 a and the buffer oxide layer 116, and thesecond transmission supplementary pattern 220 a is interposed betweenthe blocking pattern 218 a and the upper oxide layer 135. The firsttransmission supplementary pattern 216 a is made of an insulator havinga refractive index between a refractive index of the buffer oxide layer116 and a refractive index of the blocking pattern 218 a. The secondtransmission supplementary pattern 220 a is made of an insulator havinga refractive index between a refractive index of the upper oxide layer135 and a refractive index of the blocking pattern 218 a. The bottom andtop surfaces of the first transmission supplementary pattern 216 a arepreferably in direct contact with the top surface of the blockingpattern 218 a and the bottom surface of the upper oxide layer 135,respectively. The first transmission supplementary pattern 216 a, theblocking pattern 218 a, and the second transmission supplementarypattern 220 a have sidewalls aligned to one another.

If the buffer oxide layer 116 and the upper oxide layer 135 are made ofsilicon oxide and the blocking pattern 218 a is made of silicon nitride,the first and second transmission supplementary patterns 216 a and 220 aare made of an insulator having a higher refractive index than thesilicon oxide and a lower refractive index than the silicon nitride.Preferably, the first and second transmission supplementary patterns 216a and 220 a are made of, for example, silicon oxynitride.

Gate spacer 225 is disposed on one sidewall of the gate electrode 112opposite to the diode region “a”. The gate spacer 225 includes first,second, and third spacers 216 b, 218 b, and 220 b which are stacked inthe order named. The first spacer 216 b and the first transmissionsupplementary pattern 216 a are made of the same material, and the thirdspacer 220 b and the second transmission supplementary pattern 220 a aremade of the same material.

As previously described in the first embodiment, an impurity-doped layer127 is disposed in the transistor active region “b” adjacent to one sideof the gate electrode 112 opposite to the diode region “a”. Theimpurity-doped layer 127 may include lightly and heavily doped layers114 and 126 to have an LDD or DDD structure.

In the above-described image sensor, a first transmission supplementarypattern 216 a is interposed between the buffer oxide layer 116 and theblocking pattern 218 a to reduce the difference between the refractiveindex of the buffer oxide layer 116 and the refractive index of theblocking pattern 218 a. The second transmission supplementary pattern220 a is interposed between the blocking pattern 218 a and the upperoxide layer 135 to reduce the difference between the refractive index ofthe blocking pattern 218 a and the refractive index of the upper oxidelayer 135. Accordingly, the transmission coefficient of the externallight passing the layers 116, 216 a, 218 a, 220 a, and 135 on thephotodiodes 104 and 106 increases. As a result, the loss of the lightimpinging on the photodiodes 104 and 106 is reduced to enhance thephotosensitivity of the image sensor.

FIG. 6 and FIG. 7 are cross-sectional views for showing a method offorming the image sensor shown in FIG. 5.

Referring to FIG. 6, device isolation layer 102, photodiodes 104 and106, channel doping layer 108, gate electrode 112, lightly doped layer114, and buffer oxide layer 116 are formed using the same manner asdescribed in the first embodiment.

The first transmission supplementary layer 216, the blocking layer 218,and the second transmission supplementary layer 220 are sequentiallyformed on a substrate 100 including the buffer oxide layer 116.Preferably, the layers 216, 218, and 220 are conformally formed.

A photoresist pattern 222 is formed on the second transmissionsupplementary layer 220 to cover a diode region “a”. In this case, thesecond transmission supplementary layer 220 over the transistor activeregion “b” is exposed. The photoresist pattern 220 may laterally extendto partially cover one sidewall and the top surface of the gateelectrode 112 adjacent to the diode region “a”.

Referring to FIG. 7, using the photoresist pattern 222 as an etch mask,the second transmission supplementary layer 220, the blocking layer 218,and the first transmission supplementary layer 216 are successivelyanisotropically etched to form the first transmission supplementarypattern 216 a, the blocking pattern 218 a, and the second transmissionsupplementary pattern 220 a which are sequentially stacked on the dioderegion “a” and to form the gate spacer 225 on one sidewall of the gateelectrode 112. The gate spacer 225 includes first, second, and thirdspacers 216 b, 218 b, and 220 b, which are stacked in the order named.The first, second, and third spacers 216 b, 218 b, and 220 b are made ofthe same material as the first transmission supplementary pattern 216 a,the blocking pattern 218 a, and the second transmission supplementarypattern 220 a, respectively.

Following formation of the patterns 216 a, 218 a, and 220 a and thespacer 225, the photoresist pattern 222 is removed.

An upper oxide layer 135 shown in FIG. 5 is formed. Preferably, theupper oxide layer 135 is made of silicon oxide. As previously describedin the first embodiment, the upper oxide layer 135 may include asilicide barrier layer made of silicon oxide and/or at least oneinterlayer oxide layer.

In the above-described method, the photodiodes 104 and 106 are notexposed to plasma of an etch process during formation of the patterns216 a, 218 a, and 220 a on the photodiodes 104 and 106. Thus, thephotodiodes 104 and 106 may be protected from plasma damage to preventtheir degradation.

The first transmission supplementary pattern 216 a reduces thedifference between a refractive index of the blocking pattern 218 a anda refractive index of the buffer oxide layer, and the secondtransmission supplementary pattern 220 a reduces the difference betweenthe refractive index of the blocking pattern 218 a and the refractiveindex of the upper oxide layer 135. Accordingly, the loss of theexternally incident light is reduced to enhance a photosensitivity ofthe image sensor.

Embodiment 3

In yet another embodiment, a modified version of the transmissionsupplementary layer according to the first embodiment will now bedescribed. Therefore, same numerals denote the same elements throughoutthe first and third embodiments.

FIG. 8 is a cross-sectional view of an image sensor according to anotherembodiment of the present invention.

Referring to FIG. 8, blocking pattern 318 a is disposed on the bufferoxide layer 116 to cover a diode region “a”. Upper oxide layer 135 isdisposed on the blocking pattern 318 a to cover the diode region “a”.The blocking pattern 318 a is made of an insulator to preventpenetration of metallic elements. Preferably, the blocking pattern 318 ais made of, for example, silicon nitride. As previously described in thefirst and second embodiments, the buffer oxide layer 116 and the upperoxide layer 135 are preferably made of silicon oxide.

Transmission supplementary layer 316 a is disposed between the blockingpattern 318 a and the buffer oxide layer 116. Bottom and top surfaces ofthe penetration supplementary pattern 316 a are preferably in directcontact with the top surface of the buffer oxide layer 116 and thebottom surface of the blocking pattern 318 a, respectively.

The transmission supplementary pattern 316 a is made of an insulatorhaving a refractive index between the refractive index of the bufferoxide 116 and the refractive index of the blocking pattern 318 a.Particularly, if the blocking pattern 318 a is made of silicon nitrideand the upper oxide layer 116 is made of silicon oxide, the transmissionsupplementary pattern 316 a is preferably made of an insulator having arefractive index higher than the silicon oxide and lower than thesilicon nitride. Preferably, the transmission supplementary pattern 316a is made of, for example, silicon oxynitride.

A gate spacer 325 is disposed on one sidewall of the gate electrode 112opposite to the diode region “a”. The spacer 325 includes a first spacer316 b and a second spacer 318 b which are stacked in the order named.The first spacer 316 b is made of the same material as the transmissionsupplementary pattern 316 a, and the second spacer 318 b is made of thesame material as the blocking pattern 318 a.

In the above-described image sensor, the transmission supplementarypattern 316 a is disposed between the buffer oxide layer 116 and theblocking pattern 318 a to reduce a difference between a refractive indexof the buffer oxide layer 116 and a refractive index of the blockingpattern 318 a. Accordingly, an absorption coefficient of the externallight increases to enhance the photosensitivity of the image sensor.

FIG. 9 and FIG. 10 are cross-sectional views showing a method of formingthe image sensor shown in FIG. 8.

Referring to FIG. 9, from formation of the device isolation layer toformation of the buffer oxide layer 116, the third embodiment issubstantially identical to the first and second embodiments.

The transmission supplementary layer 316 and the blocking layer 318 areconformally formed on the buffer oxide layer 116. The transmissionsupplementary layer 316 is made of an insulator having a refractiveindex between the refractive index of the buffer oxide layer 116 and therefractive index of the blocking layer 318. Particularly, if the bufferoxide layer 116 is made of silicon oxide and the blocking layer 318 ismade of silicon nitride, the transmission supplementary layer 316 ismade of an insulator having a refractive index higher than the siliconoxide and lower than the silicon nitride. Preferably, the transmissionsupplementary layer 316 is made of, for example, silicon oxynitride.

A photoresist pattern 322 is formed on the blocking layer 318 to coverphotodiodes 104 and 106 formed in the diode region “a”. In this case,the blocking layer 318 over the transistor active region “b” is exposed.The photoresist pattern 318 may laterally extend to partially cover onesidewall and the top surface of the gate electrode adjacent to thephotodiodes 104 and 106.

Referring to FIG. 10, using the photoresist pattern 322 as an etch mask,the blocking layer 318 and the transmission supplementary layer 316 aresuccessively anisotropically etched to form the transmissionsupplementary pattern 316 a and the blocking pattern 318 a which aresequentially stacked on the photodiodes 104 and 106 and to form a gatespacer 325 on one sidewall of the gate electrode 112. In this case, thebuffer oxide layer 116 may act as an etch-stop layer. The gate spacer325 includes a first spacer 316 b and a second spacer 318 b which arestacked in the order named. The first spacer 316 b is made of the samematerial as the transmission supplementary pattern 316 a, and the secondspacer 318 b is made of the same material as the blocking pattern 318 a.

The photoresist pattern 322 is removed from the substrate 100.

The upper oxide layer 135 shown in FIG. 8 is formed. As previouslydescribed in the first and second embodiments, the upper oxide layer 135may include a silicide barrier layer made of silicon oxide and/or atleast one interlayer oxide layer.

In the above-described method, the photodiodes 104 and 106 are notexposed to plasma during an etch process for forming the transmissionsupplementary pattern 316 a and the blocking pattern 318 a. Therefore,it is possible to prevent degradation of the photodiodes 104 and 106.Since the transmission supplementary pattern 316 is disposed between thebuffer oxide layer 116 and the blocking pattern 318 a, the differencebetween the refractive index of the buffer oxide layer 116 and therefractive index of the transmission supplementary pattern 316 may bereduced to enhance a photosensitivity of the image sensor.

The image sensor according to the present invention is not limited toCMOS-type image sensors. That is, the teachings of the present inventionmay be applied to all images sensors using at least one of thephotodiodes 104 and 106.

As explained so far, the transmission supplementary pattern interposesbetween the blocking pattern covering the photodiode and the upper oxidelayer and/or between the blocking pattern and the buffer oxide layer.The transmission supplementary pattern reduces the difference betweenthe refractive index of the blocking pattern and the refractive of theupper oxide layer and/or the difference between the refractive index ofthe blocking pattern and the refractive index of the buffer oxide layer.Thus, increasing the absorption coefficient of the light will enhancethe photosensitivity of the image sensor.

Although the present invention has been described with reference to thepreferred embodiments thereof, it will be understood that the inventionis not limited to the details thereof. Various substitutions andmodifications have been suggested in the foregoing description, andother will occur to those of ordinary skill in the art. Therefore, allsuch substitutions and modifications are intended to be embraced withinthe scope of the invention as defined in the appended claims.

1. An image sensor comprising: a photodiode formed on a substrate; a buffer oxide layer covering the photodiode; a blocking layer disposed on the buffer oxide layer to cover the photodiode; an upper oxide layer covering the blocking layer; and a transmission supplementary layer interposed between the upper oxide layer and the buffer oxide layer to cover the photodiode, the transmission supplementary layer having a refractive index between the refractive index of the blocking layer and at least one reflective index selected from the reflective indexes of the buffer oxide layer and upper oxide layer.
 2. The image sensor as recited in claim 1, wherein the buffer oxide layer and upper oxide layer are made of silicon oxide and the blocking layer is made of silicon nitride, and the transmission supplementary layer is made of an insulator having a refractive index higher than the silicon oxide and lower than the silicon nitride.
 3. The image sensor as recited in claim 2, wherein the transmission supplementary layer is made of silicon oxynitride.
 4. The image sensor as recited in claim 1, wherein the photodiode comprises: an N-type photodiode formed in the substrate; and a P-type photodiode formed at a surface of the substrate on the N-type photodiode.
 5. The image sensor as recited in claim 1, wherein the transmission supplementary layer is interposed between the blocking layer and the upper oxide layer.
 6. The image sensor as recited in claim 1, wherein the transmission supplementary layer is interposed between the buffer oxide layer and the blocking layer.
 7. The image sensor as recited in claim 1, wherein the transmission supplementary layer comprises: a first transmission supplementary layer interposed between the buffer oxide layer and the blocking layer; and a second transmission supplementary layer interposed between the blocking layer and the upper oxide layer.
 8. A method of forming an image sensor comprising: forming a photodiode on a substrate; forming a buffer oxide layer to cover the photodiode; forming a blocking layer on the buffer oxide layer to cover the photodiode; forming an upper oxide layer to cover the blocking layer; and forming a transmission supplementary layer between the upper oxide layer and the buffer oxide layer to cover the photodiode, the transmission supplementary layer having a refractive index between the refractive index of the blocking layer and at least one reflective index selected from the reflective indexes of the buffer oxide layer and upper oxide layer.
 9. The method as recited in claim 8, wherein the buffer and upper oxide layers are made of silicon oxide and the blocking layer is made of silicon nitride, and the transmission supplementary layer is made of an insulator having a refractive index higher than the silicon oxide and lower than the silicon nitride.
 10. The method as recited in claim 9, wherein the transmission supplementary layer is made of silicon oxynitride.
 11. The method as recited in claim 8, wherein the formation of the photodiode comprises: forming an N-type photodiode in a predetermined region of the substrate; and forming a P-type photodiode at a surface of the substrate on the N-type photodiode.
 12. The method as recited in claim 8, wherein the formation of the transmission supplementary layer and the blocking layer comprises: forming the blocking layer on the buffer oxide layer; and forming the transmission supplementary layer on the blocking layer.
 13. The method as recited in claim 8, wherein the formation of the transmission supplementary layer and the blocking layer comprises: forming the transmission supplementary layer on the buffer oxide layer; and forming the blocking layer on the transmission supplementary layer.
 14. The method as recited in claim 8, wherein the formation of the transmission supplementary layer and the blocking layer comprises: forming a first transmission supplementary layer on the buffer oxide layer to cover the photodiode; forming the blocking layer on the first transmission supplementary layer; and forming a second transmission supplementary layer on the blocking layer to cover the photodiode, wherein the transmission supplementary layer comprises the first and second transmission supplementary layers.
 15. An image sensor comprising: a photodiode formed on a substrate; a buffer oxide layer covering the photodiode; a blocking layer disposed on the buffer oxide layer to cover the photodiode; an upper oxide layer covering the blocking layer; and a transmission supplementary layer interposed between the blocking layer and the buffer oxide layer to cover the photodiode, the transmission supplementary layer having a refractive index between the refractive index of the blocking layer and at least one reflective index selected from the reflective indexes of the buffer oxide layer and upper oxide layer.
 16. The image sensor as recited in claim 15, wherein the buffer oxide layer and upper oxide layer are made of silicon oxide and the blocking layer is made of silicon nitride, and the transmission supplementary layer is made of an insulator having a refractive index higher than the silicon oxide and lower than the silicon nitride.
 17. The image sensor as recited in claim 16, wherein the transmission supplementary layer is made of silicon oxynitride.
 18. The image sensor as recited in claim 15, wherein the photodiode comprises: an N-type photodiode formed in the substrate; and a P-type photodiode formed at a surface of the substrate on the N-type photodiode. 